1. Field of the Invention
The present invention relates in general to marine engines and more particularly to exhaust systems for marine engines.
2. Description of the Related Art
Marine engines typically use water-cooled exhaust systems in which water already circulated through the engines cooling system is utilized to cool exhaust pipes and to lower engine compartment temperatures. To accomplish this, most marine engines use double walled pipes with the exhaust passing through the inside pipe and the spent cooling water flowing in the cavity between the two pipes. At some point in the system the inner wall terminates and the water and exhaust mix and exit the exhaust system together. It is important that the water and exhaust particles exit the system rather than being ingested into the engine to prevent the phenomenon known as hydro-locking. Hydro-locking is essentially the ingestion of water into the cylinders of the engine. Since the water can not be compressed or ignited, the pistons essentially "lock up" and the engine seizes.
In an internal combustion engine, air and exhaust move in an unsteady manner due to many factors such as intake and exhaust valves opening and closing, differing throttle positions, continually changing pressures and temperatures in the engine, intake and exhaust system shape and flow patterns. The conventional nomenclature for this phenomenon is "unsteady gas dynamics".
Within the ducting or tubing system of internal combustion engines there are two types of finite-amplitude waves that can occur, a compression wave and an expansion wave. A compression wave is always a positive pressure wave with greater pressure than atmospheric pressure and an expansion wave has lower pressure than atmospheric pressure. Compression waves always move particles in the direction of their propagation and expansion waves always move particles in the direction opposite their propagation. Pressure waves and particle waves do not necessarily move at the same speed.
In an internal combustion engine, when the exhaust valve is opened and the piston is on the exhaust stroke, a compression wave is formed that moves from the exhaust valve toward the end of the exhaust pipe and subsequently into the atmosphere. As the compression wave leaves the exhaust pipe a reflected expansion wave is formed that moves back toward the exhaust valve. As explained previously, this expansion wave moves particles opposite the direction of wave travel so the particle flow is toward the open end of the pipe or outlet end of the exhaust system into the atmosphere.
In an optimally tuned internal combustion engine, both the compression waves and expansion waves can effect and contribute to exhaust particle movement. In the case of marine engines, these waves contribute to move the mixture of spent cooling water and exhaust gas particles as they travel within the exhaust system out of the exhaust system. Therefore, in an internal combustion engine, it can be said that exhaust gas particles and spent cooling water move out of an internal combustion engine exhaust pipe to the underwater environment due to the phenomenon that compression waves will move particles in the direction of propagation and expansion waves will move particles in the direction opposite their propagation. This phenomenon occurs in a pipe with openings at both ends (i.e. with an exhaust pipe open to the underwater environment at one end and with a valve open to the combustion chamber at the other end).
In correctly designed internal combustion operation, at steady state conditions, the expansion waves may propagate towards the exhaust valve when it is open. The negative pressure expansion wave actually enhances combustion cylinder exhaust flow during the exhaust stroke when the negative pressure condition encounters the positive pressure being generated by the piston action. This allows the gas particles to continue to move out of the engine, toward the end of the exhaust pipe, and then into the atmosphere. This phenomenon is basically balanced in the engine when running at quasi steady state (i.e. when acceleration or deceleration is not radical).
However when the throttle is snapped shut quickly from high rpm there is a "lag" time in the exhaust system where previously created expansion waves are traveling. During this lag time the exhaust valve is not open in concert with the returning expansion waves. In this condition the exhaust valve behaves like a closed end pipe and the expansion waves reflect and change direction. This change in direction changes the direction of both the expansion negative pressure wave and the gas particle movement. Instead of the gas particles moving toward the atmosphere (against the expansion wave) via the end of the exhaust pipe, they move toward the closed exhaust valve (again in the opposite direction of the expansion wave). Moreover, when the expansion wave reaches the end of the pipe (and thus the atmosphere) it reflects back as a compression wave moving opposite the desired direction. Both waves then drive particle flow back towards the exhaust valve. This phenomenon will cause gas particles and water in the exhaust stream to move towards the manifold, valve and cylinder into the previously dry exhaust pipe. If the engine is accelerated or decelerated (typically only decelerated) quickly causing a reverse of the desired gas dynamics, there is potential for sufficient water to be introduced into the exhaust manifold whereby a hydro-locking of the engine could occur. Such an occurrence can result in severe or catastrophic engine failure.
Marine engine manufacturers have not designed a fully capable solution to this problem and boat owners are normally faced with replacing engines that have ingested water in such a manner. Current design techniques increase the height of the exhaust riser in an effort to have more suction head in the riser than is produced by the negative pressure wave. This does not address the basic problem of water ingestion, but only creates a greater distance for the water to travel before entering the exhaust valve. Another technique is to form a sudden change in the cross sectional area of the exhaust pipe just after the water and exhaust mix in an effort to reflect the expansion wave back to the open exhaust end, prior to water and particle flow reaching the exhaust valve area. These methods have had limited success in the past, mostly due to the limitations in engine compartment height available from boat manufacturers and insufficient area available for wave reflection.
Another problem that is encountered by boat owners is that when a carbureted engine is turned off, the act of turning the key to the off position opens the circuit to the ignition coil thus eliminating the spark to the spark plug. The engine then coasts down to a stop. During this coast down period, air and fuel are still drawn through the engine and raw fuel and air are drawn into the intake system into the cylinder then expelled into the exhaust system. Often, due to low octane fuel or a hot spot in the combustion chamber, the engine will "diesel" or run backwards just prior to coming to a stop. The engine can run backwards for several revolutions until the raw fuel in the exhaust system is spent. While the engine is running backwards, the exhaust system assumes the role of an intake system and the air in the exhaust system is pulled back into the cylinder along with the water in the exhaust system, thereby causing the engine to be hydro-locked.
Accordingly, there is a need in the art for an anti ingestion device which can be used in conjunction with an internal combustion engine to reliably prevent the back flow of water and particles into the intake manifold.